1. Field of the Disclosure
Exemplary embodiments of the present disclosure generally relate to an event input module, and more particularly to an event input module configured to be used for time synchronization among PLCs (Programmable Logic Controllers).
2. Discussion of the Related Art
Integration, automation and remote control monitoring to minimize human intervention have been lately promoted in power transmission and distribution fields, which have become enabled by replacement of conventional electric devices with communication function-equipped, microprocessor-based intellectual electronic devices.
Many electric devices inside substations require time synchronization for accurate and effective information transfer, for which a variety of methods are being used for time synchronization of devices applicable to substation automation systems that include GPS (Global Positioning System), NTP (Network Time Protocol), SNTP (Simple Network Time Protocol), IRIG (Inter-Range Instrumentation Group)-B, and IEEE (Institute of Electrical and Electronics Engineers) 1588. One of the most generalized methods, among the abovementioned methods, is to perform synchronization with outside standard time using the GPS, the method of which however has a disadvantage in that a GPS module must be mounted, and expandability is limited to incur lots of costs.
Furthermore, the methods such as NTP and SNTP for realizing software time synchronization cannot perform the time synchronization if a difference between a current time and a time to be synchronized is too great, and even if the synchronization is performed, accuracy disadvantageously deteriorates to make them difficult to be applied to merging units of power system.
Meanwhile, the event input module, which is based on DC input module widely used in the PLC, is a data recording device functioning to collect input event information (time and state) in a predetermined resolution through a record relative to outside event. The event input module basically detects an event when an outside input is changed from ON to OFF and when the outside input is changed from OFF to ON, and stores a relevant time in internal memory. The event information is stored in order of being generated, which plays an important role in such fields as power generation, power transmission, power distribution and process plant.
FIG. 1 is a block diagram illustrating synchronization of PLC systems according to prior art.
A GPS (Global Positioning System) receiver (200) in the conventional PLC system receives a time data from a satellite (100) and outputs the data in a NMEA-0183 message. A signal outputted in an RS232 format goes through a converter (300) configured to convert to an RS485 signal for time synchronization between two PLC systems (400, 500). The RS485 signal is capable of multidrop, such that the signal is transmitted to PLC CPU (Central Processing Unit, 410, 510) modules of two systems through RS485 communication modules (420, 520) mounted on the two systems.
Each PLC CPU (410, 510) maintains the time data transmitted from the GPS receiver (200), and event input modules (430, 530) of each system store a relevant time in memories of the event input modules (430, 530) the moment an event input generated from a sensor output unit (700) is detected. A user checks the event data by connecting to the PLC CPU (410, 510) of PLC systems (400, 500) through an application terminal (600).
The GPS receiver (200) can output time by way of a variety of types and requires an interface of serial type for connection with the PLC systems (400, 500). Conventionally, an NMEA (National Marine Electronics Association)-0183 was used, and NMEA-0183 defines electrical transmission standards of RS-232, RS-422.
Event detection condition of event input modules (430, 530) is when a signal of the sensor output unit (700) is changed from low to high or from high to low. When a set event condition is detected, an LED (Light Emitting Diode) of the event input modules (430, 530) is displayed with an event state. The two PLC systems (400, 500) require time synchronization in the system configured as in FIG. 1.
FIG. 2 is a block diagram modeling a time delay of FIG. 1.
Referring to FIG. 2, Δt1 defines a transmission delay from the GPS receiver (200) to an RS232-RS485 converter (300). Δt2 and Δt2′ define a transmission delay from the converter (300) the communication modules (420, 520) and Δt3 and Δt3′ define a process delay of data processed by the communication modules (420, 520). Δt4 and Δt4′ define a transmission delay from the communication modules (420, 520) to the CPU modules (410, 510), and Δt5 and Δt5′ define a process delay of data processed by the CPU modules (410, 510). Δt6 and Δt6′ define a transmission delay from the CPU modules (410, 510) to the event input modules (430, 530), Δt7 and Δt7′ define a process delay of signal processed by the event input modules (430, 530). Furthermore, Δt8 and Δt8′ are a transmission delay from the sensor output unit (700) to the event input modules (430, 530).
A total time delay at the systems (400, 500) illustrated in FIG. 1 may be expressed by the following Equations 1 and 2.
                              ∑                      i            =            1                    8                ⁢                                  ⁢                  Δ          ⁢                                          ⁢          ti                                    [                  Equation          ⁢                                          ⁢          1                ]                                          ∑                      i            =            1                    8                ⁢                                  ⁢                  Δ          ⁢                                          ⁢                      ti            ′                                              [                  Equation          ⁢                                          ⁢          2                ]            
A user expects that delay elements of two systems (400, 500) are shown with a same value for time synchronization of two systems (400, 500). However, the delay elements of two systems cannot become identical to have a predetermined error, which is generally determined by a standard of a product for time synchronization.
A major delay element that generates a time synchronization error between two systems in the convention configuration is from Δt3(Δt3′) to Δt5(Δt5′), and it is due to the fact that the time information coming through the GPS receiver (200) is inevitably under the influence of the delay element generated from the communication modules (420, 520) and a scan period of PLC program performed by the CPU modules. This error is such that the time synchronization develops an important application as mutually different times are recorded when an event generated through the sensor output unit (700) is detected by the event input modules.